Oscillation circuits which provide periodically oscillating signals or clocks are essential building blocks of communication systems.
Please refer to FIG. 1 illustrating a multi-band differential oscillation circuit 10 according to a prior art. The oscillation circuit 10 has two nodes ns1 and ns2 as two oscillation nodes which output a differential pair of oscillation signals. The oscillation circuit 10 operates between a power voltage VDD and a ground voltage VSS, and includes two transistors M1 and M2, inductors L1 and L2, capacitors CV1 and CV2, as well as K capacitor switch units Bp(1) to Bp(K). The transistors M1 and M2 form a feedback amplifier; the inductors L1 and L2, the capacitors CV1 and CV2 and the capacitor switch units Bp(1) to Bp(K) form an LC (inductance-capacitance) loading of the amplifier. The capacitors CV1 and CV2 are variable capacitors with their capacitance controlled by an adjusting signal Sfc. The k-th capacitor switch unit Bp(k) of the capacitor switch units Bp(1) to Bp(K) includes a pair of capacitor C1(k) and C2(k), and a pair of transistors T1(k) and T2(k) as switches. The capacitor C1(k) and the transistor T1(k) are coupled between the nodes ns1, n1(k) and the ground voltage VSS; the capacitor C2(k) and the transistor T2(k) are coupled between the nodes ns2, n2(k) and the ground voltage VSS; and gates of the transistors T1(k) and T2(k) are controlled by a corresponding control signal Sp(k).
For the oscillation signals provided by the oscillation circuit 10, the oscillation frequency depends on a product of total inductance and total capacitance L_total*C_total; the oscillation frequency is calculated by: 1/(2*pi*sqrt(L_total*C_total)), where pi is the constant ratio between circle's circumference to diameter. The total inductance L_total is dominated by inductance of the inductors L1 and L2, and the total capacitance C_total depends on capacitance of the capacitors CV1 and CV2, as well as capacitance provided by the capacitor switch units Bp(1) to Bp(K). When the control signal Sp(k) equals the power voltage VDD to turn on the transistors T1(k) and T2(k), the capacitors C1(k) and C2(k) respectively provide capacitance to the nodes ns1 and ns2, thus the total capacitance C_total increases; on the contrary, when the control signal Sp(k) equals the ground voltage VSS to turn off the transistors T1(k) and T2(k), only the parasitic components of capacitors C1(k) and C2(k) contribute to the total capacitance C_total.
When the control signals Sp(1) to Sp(K) of the capacitor switch units Bp(1) to Bp(K) remain fixed, the oscillation frequency of the oscillation circuit 10 changes along a band, such as a band BF(j) shown in FIG. 1, following change of the analog adjusting signal Sfc. By changing the digital control signal Sp(k) corresponding to the capacitor switch unit Bp(k), the oscillation frequency provided by the oscillation circuit 10 switches to different bands BF(0), BF(1), . . . , BF(j) to BF(J). For example, if all the control signals Sp(1) to Sp(K) equal the ground voltage VSS, the oscillation frequency of the oscillation circuit 10 varies along the band BF(0) following tuning of the adjusting signal Sfc. If a given control signal Sp(k) conducts its corresponding capacitor switch unit Bp(k) to contribute capacitance, the oscillation frequency varies along the band BF(1) of lower frequency, etc. The frequencies covered by the bands BF(0) to BF(J) span the oscillation frequency range of the oscillation circuit 10.
To evaluate performance of an oscillation circuit, the quality factor Q is an important performance reference. For an oscillation circuit with higher Q, its oscillation frequency converges with narrower dispersion bandwidth, and consumes less power. The quality factor Q is limited by equivalent resistance serially coupled to capacitance (in addition to equivalent resistance serially coupled to the inductance); the higher the resistance is, the lower the quality factor Q becomes. In the oscillation circuit 10 of prior art, when the transistors T1(k) and T2(k) conduct so the capacitors C1(k) and C2(k) contribute capacitance, source-drain turn-on resistance of the transistors T1(k) and T2(k) will be serially coupled to the capacitors C1(k) and C2(k) respectively at the nodes n1(k) and n2(k), and thus the quality factor of the oscillation circuit 10 is lowered.
To improve Q of the oscillation circuit 10, the transistors T1(k) and T2(k) in the capacitor switch unit Bp(k) must be implemented by transistors of larger (wider) sizes to reduce turn-on resistance of the transistors T1(k) and T2(k). However, larger transistors T1(k) and T2(k) suffer form greater parasitic capacitance which affects the total capacitance C_total of the oscillation circuit 10, so the oscillation frequency range of the oscillation circuit 10 is limited, and the oscillation frequency range can not be effectively increased. If smaller transistors T1(k) and T2(k) are adopted, not only the quality factor Q degrades, but also the phase noise of the oscillation circuit 10 raises to cause severe oscillation jitter.